Multi-cell processing architectures for modeling and impairment compensation in multi-input multi-output systems

ABSTRACT

A method for predistortion comprising receiving a plurality of input signals forming a multiple-input signal in a multiple-input multiple-output system, generating a pre-distorted multiple-input signal from the received multiple-input signal, generating a multiple-output signal by feeding the pre-distorted multiple-input signal into a multiple-input and multiple-output transmitter, estimating impairments generated by the multiple-input and multiple-output transmitter, the impairments comprising nonlinear crosstalk between distinct ones of the plurality of input signals; and adjusting the pre-distorted multiple-input signal to compensate for the estimated impairments.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims priority to,U.S. Ser. No. 15/483,382 filed Apr. 10, 2017 and which is a continuationof U.S. Ser. No. 14,319,421 filed Jun. 30, 2014 which is aContinuation-in part of U.S. Ser. No. 12/780,455 filed May 14, 2010 (nowU.S. Pat. No. 8,767,857) which claims the benefits of the filing date ofU.S. provisional patent application No. 61/213,176 filed on May 14,2009. U.S. Ser. No. 14,319,421 is a further continuation in part of U.S.Serial No 13/563,621 filed Jul. 31, 2012 which is a Continuation-in-Partof and claims the benefit of the filing date of U.S. patent applicationSer. No. 13/105,852, filed May 11, 2011, all of which are in theirentirety herein incorporated by reference.

TECHNICAL FIELD

This present disclosure relates to the field of wireless communications,and more specifically, to the distortions and impairment's correctionsof Multiple Input Multiple Output (MIMO) systems with linear andnonlinear components and unwanted interactions and correlations betweenmultiple input signals.

BACKGROUND

MIMO refers to a system with multiple inputs and multiple outputs. Thedefinition of MIMO system is extended to wireless communicationtopologies in which multiple modulated signals, separated in frequencyor space domain, are simultaneously transmitted through asingle/multiple branch radiofrequency (RF) front-end.

MIMO systems, with modulated signals separated in space domain, refer towireless topologies with multiple branches of RF front-ends, with allbranches simultaneously involved in signal transmission. These types ofMIMO systems are considered as Multi-branch MIMO systems.

MIMO systems, with modulated signals separated in frequency domain,refer to systems where multiple signals modulated in different carrierfrequencies are concurrently transmitted through a single branch RFfront-end. These types of MIMO systems are considered as Multi-frequencyMIMO systems. Examples of multi-frequency MIMO systems are concurrentdual-band and multi-carrier transmitters. The system in frequency domaincomprises two independent baseband signals as the multiple inputs andtwo up-converted and amplified signals at two carrier frequencies as themultiple outputs. In fact, this type of MIMO system uses a single branchRF front-end to transmit multiple signals.

RF MIMO systems are composed of linear and nonlinear components and/orsub-blocks which may results in signal quality degradation. For example,the power amplifier (PA) is one of the main building blocks of the RFfront-end that has a significant nonlinear behavior. This nonlinearrelation between the input signal and the amplified output signal of thetransmitter results in significant distortions on the output signal.These distortions significantly degrade the output signal's quality andresult in poor data communications. In this regard, different techniquesto compensate for these distortions were proposed in order to improvethe linearity of the RF radio front-end.

Also, there are unwanted and unavoidable interactions and correlationsbetween the different signals in a MIMO system. These interactions arecombined with the linear and nonlinear distortions in each branch of theMIMO system to generate more complex distortion effects, whichconsiderably degrade the performance of the MIMO system. The effect ofthese complex distortions cannot be eliminated or reduced withconventional signal processing algorithms applied to Single Input SingleOutput (SISO) systems.

Therefore, there is a need for a signal processing technique for MIMOsystems that compensates for any distortion, interactions, and crosstalkin the system in order to improve the signal quality of the transmissionlink.

SUMMARY

MIMO systems require special processing architectures, which compensatefor the complex distortions in order to transmit and/or receive goodquality signals. Processing architectures that are conventionally usedwith SISO system do not consider the interactions between the differentinput signals of the MIMO systems. This requires a more complexprocessing architecture that considers the effect of interaction betweenthe multiple input signals.

Therefore, according to the present invention, there is provided amethod for multiple-input multiple-output impairment pre-compensationcomprising: receiving a multiple-input signal; generating apre-distorted multiple-input signal from the received multiple-inputsignal; generating a multiple-output signal by feeding the pre-distortedmultiple-input signal into a multiple-input and multiple-outputtransmitter; estimating impairments generated by the multiple-input andmultiple-output transmitter; and adjusting the pre-distortedmultiple-input signal to compensate for the estimated impairments.

According to the present invention, there is also provided apre-compensator for use with a multiple-input and multiple-outputtransmitter, comprising: a multiple-input for receiving a multiple-inputsignal; a matrix of pre-processing cells for generating a pre-distortedmultiple-input signal from the received multiple-input signal; and amultiple-output for feeding the pre-distorted multiple-input signal tothe multiple-input and multiple-output transmitter; wherein thepre-processing cells are configured so as to estimate impairmentsgenerated by the multiple-input and multiple-output transmitter andadjust the pre-distorted multiple-input signal to compensate for theestimated impairments.

The present invention further relates to a compensator for use with amultiple-input and multiple-output transmitter, comprising: amultiple-input for receiving a multiple-input signal; a matrix ofprocessing cells for generating a distorted multiple-input signal fromthe received multiple-input signal; and a multiple-output for feedingthe pre-distorted multiple-input signal; wherein the pre-processingcells are configured so as to estimate impairments generated by themultiple-input and multiple-output transmitter and adjust thepre-distorted multiple-input signal to compensate for the estimatedimpairments.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described by way of example onlywith reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a Multiple Input Multiple Output (MIMO)system with a pre-compensator;

FIG. 2 is a block diagram of an example of pre-distortion linearizationtechnique in the form of a cascade of a signal processing block and atransmitter;

FIG. 3 is a graph of measured output spectrums of a Single Input SingleOutput (SISO) transmitter with and without digital pre-distortionlinearization technique;

FIG. 4 is a block diagram of a dual branch MIMO transmitter;

FIG. 5 is a block diagram of a MIMO system with a digitalpre-compensator having four cells and a dual branch MIMO transmitter;

FIG. 6 is a graph of measured output spectrum of a dual branch MIMOtransmitter with and without the pre-compensation signal processingtechnique and for the linear MIMO transmitter;

FIG. 7 is a block diagram of a MIMO system with a number N of RF pathscascaded with a digital pre-compensator;

FIG. 8 is a block diagram of a dual-band transmitter with dual inputsand single branch nonlinear transmitter;

FIG. 9 is a graph of the output signal from a nonlinear transmitterwhich shows intra-band and inter-band distortions;

FIG. 10 is a block diagram of a system comprising a multi-cellprocessing pre-compensator cascaded with a dual-band transmitter;

FIG. 11 is a block diagram of a system comprising a multi-cellprocessing pre-compensator cascaded with a multi-carrier transmitter;and

FIG. 12 is a diagram of a processing cells matrix for MIMO systems.

DETAILED DESCRIPTION

Linear and nonlinear distortions are the main sources of performancedegradation in RF front-ends. These distortions affect the signalquality and lead to an unacceptable data communication. In situationswhere both linear and nonlinear distortions are present simultaneously,the conventional signal processing algorithms are not able to eliminateand compensate for these distortions. To overcome this drawback, thereis provided a signal processing to simultaneously compensate for bothlinear and nonlinear distortions and impairments.

Referring to FIG. 1, there is shown an example of a system 100, havingmultiple inputs 110 and multiple outputs 140, comprising a MultipleInput Multiple Output (MIMO) RF front-end 130 having degradedperformance due to the nonlinear behavior of the integrated RF PAs andthe coupling effects between the multiple RF paths. In this case, theMIMO RF front-end 130 suffers from a joint effect of linear andnonlinear distortions. A MIMO pre-compensator processing block 120 iscascaded to the MIMO RF front-end 130 to compensate for all linear andnonlinear distortions of the MIMO system 100.

Referring now to FIG. 2, there is shown an example of pre-distortionlinearization 200 for a Single Input Single Output (SISO) transmitter,which may be used to illustrate the basic concept behind signalpre-processing methods. Pre-distortion linearization 200 includes asignal processing block 220, which pre-processes the input basebandsignal 210 to generate a pre-distorted baseband signal 230. Then thepre-distorted signal 230 is supplied to the nonlinear transmitter 240 toproduce an output signal 250. Both the signal processing block 220 andthe transmitter 240 have nonlinear behavior; however, the cascade ofboth block 220 and transmitter 240 has a linear response. Therefore, theoutput signal 250 is a linear amplified version of the input basebandsignal 210. If f(x) is a function that models the nonlinear behavior ofthe transmitter 240 extracted using the baseband input signal (z) 230,and the equivalent complex envelope of sampled RF signal at the outputof the transmitter (y) 250, the pre-distortion function of the signalprocessing block 220, g(x), has to satisfy the following set ofrelations:

y=ƒ(z) and z=g(x)ƒ(g(x))=G ₀ x  Equation 1where G₀ is the linear or small-signal gain of the transmitter 240.

FIG. 3 shows the output spectrum of the nonlinear transmitter 240presented in FIG. 2 with and without using the signal processing block220 (with linearization and without linearization, respectively in FIG.3). The use of the signal processing block 220 significantly reduces theout-of-band distortion of the signal and improves quality of the signal.

In transmitters for multi-branch MIMO systems, the transmitter's linearand nonlinear distortions on each branch may be coupled because of theinterference and crosstalk between the multiple front-ends of thetransmitter. Indeed, crosstalk or coupling is more likely to happenbetween the paths in the case of multiple RF paths with the sameoperating frequency and power. This crosstalk phenomenon is expected tobe more significant in integrated circuit (IC) design, where the size ofthe prototype is a critical design parameter.

Referring to FIG. 4, there is shown a dual branch MIMO transmitter 420as an example of a multi-branch MIMO system 400. The dual branch MIMOtransmitter 420 comprises low pass filters 430A and 430B, up-converters435A and 435B and a local oscillator (L.O.) 440, and nonlineartransmitters 445A and 445B. The transmitters 445A and 445B exhibitnonlinear and/or linear distortion behaviors. The distortion behaviorsmay include but not limited to nonlinear power response of the activedevices such as the power amplifier, frequency response, memory effect,branch imbalance, DC and carrier offset, and/or image interference.

The crosstalk or coupling in dual branch MIMO transmitter may beclassified as linear crosstalk, 455, and/or nonlinear crosstalk, 450.The crosstalk is considered linear when the effect of the crosstalk atthe output of the transmitter 460 can be modeled as a linear function ofthe interference 460B and desired signal 460A. In other words, the inputsignals 410 affected by linear crosstalk 455 do not pass throughnonlinear components such as 445A and 445B. Conversely, the nonlinearcrosstalk 450 affects the input signals 410 before it passes throughnonlinear components such as 445A and 445B. The nonlinear crosstalkproduces undesired signal 460C at the output of the dual branch MIMOtransmitter 400. The sources of nonlinear crosstalk 450 may beinterferences in the chipsets between the different paths of the MIMOtransceiver and leakage of RF signals through the common localoscillator 440 path.

Referring now to FIG. 5, there is shown a MIMO system 500 comprising adigital pre-compensator 520 with dual inputs and dual outputs cascadedin front of a dual branch MIMO transmitter 540 similar to the oneillustrated in FIG. 4 (with components 550A, 550B, 555, 560A and 5606 ofFIG. 5 corresponding to components 435A, 435B, 440, 445A and 445B ofFIG. 4). The digital pre-compensator 520 uses a matrix of fourprocessing cells 515 in order to compensate for the dual branchnonlinearities and any crosstalk and interference (impairments) betweenthe two RF paths. The digital pre-compensator 520 comprises means, forexample the processing cells, using the input signals 530 and outputsignals 570 of the dual branch MIMO transmitter 540 to estimate anynonlinearities and interferences (impairments) and identify a properprocessing function for each of the four processing cells 515. Afteridentification, the input signals 510 are supplied to the fourprocessing cells 515 to generate and adjust the pre-distorted signals530. Then the pre-distorted signals 530 are supplied to the dual branchMIMO transmitter 540. The cascade of the digital pre-compensator 520 andthe dual branch MIMO transmitter 540 exhibit linear behavior. The outputsignals 570 are the linear amplified version of the input signals 510without the effect of the transmitter linear and nonlinear distortionsand crosstalk on the quality of the signals. Therefore, the digitalpre-compensator 520 compensate for all the linear and nonlineardistortions and crosstalk (impairments) in the different branches of theMIMO transmitter 540.

Referring to FIG. 6, there is shown the measured output spectrum of thedual branch MIMO transmitter 540 for three cases: case-1) in thepresence of −20 dB crosstalk and without using the digitalpre-compensator 520, case-2) in the presence of −20 dB crosstalk andusing the digital pre-compensator 520, and case-3) for a perfect MIMOtransmitter without any crosstalk and nonlinearities. The outputspectrum of case-2 with −20 dB crosstalk and digital pre-compensator 520is almost following the one in case-3; this demonstrates that thedigital pre-compensator 520 can compensate for the effect of bothtransmitter nonlinearities and crosstalk (impairments).

Referring to FIG. 7, there is shown an example of a system 700comprising a digital pre-compensator with multiple inputs and multipleoutputs 720, having a RF front-end 740 with a number N of outputs 750.The digital pre-compensator 720 can be modeled as a N×N matrix 725 whereeach cell of the matrix represents a processing block. For example,D_(i,j) represents the processing block between the i^(th) input signaland the j^(th) output of the digital compensator 720. The matrixrepresentation of the digital compensator block based on the inputsignals x 710 and output signals Y 730 can be expressed as follows:

$\begin{matrix}{\lbrack {{\overset{arrow}{Y}}_{1}\mspace{14mu}\ldots\mspace{14mu}{\overset{arrow}{Y}}_{i}\mspace{14mu}\ldots\mspace{14mu}{\overset{arrow}{Y}}_{N}} \rbrack = {\lbrack {A_{{\overset{arrow}{x}}_{1}}\mspace{14mu}\ldots\mspace{14mu} A_{{\overset{arrow}{x}}_{i}}\mspace{14mu}\ldots\mspace{14mu} A_{{\overset{arrow}{x}}_{N}}} \rbrack{\quad\begin{bmatrix}D_{1,1} & D_{1,2} & \ldots & \ldots & D_{1,{N - 1}} & D_{1,N} \\D_{2,1} & D_{2,1} & \ldots & \ldots & D_{2,{N - 1}} & D_{2,N} \\\vdots & \vdots & \ddots & ⋰ & \vdots & \vdots \\\vdots & \vdots & ⋰ & \ddots & \vdots & \vdots \\D_{{N - 1},1} & D_{{N - 1},2} & \ldots & \ldots & D_{{N - 1},{N - 1}} & D_{{N - 1},N} \\D_{N,1} & D_{N,2} & \ldots & \ldots & D_{N,{N - 1}} & D_{N,N}\end{bmatrix}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$where the parameters in Equation 2 are defined as:

$\begin{matrix}{\mspace{76mu}{{A_{\overset{arrow}{x}} = {\lbrack {\beta_{\overset{arrow}{x}}^{0}\mspace{14mu}\ldots\mspace{14mu}\beta_{\overset{arrow}{x}}^{q}\mspace{14mu}\ldots\mspace{14mu}\beta_{\overset{arrow}{x}}^{Q}} \rbrack\mspace{14mu}{is}\mspace{14mu}{an}\mspace{14mu} N \times {K( {Q + 1} )}\mspace{14mu}{matrix}}},{\beta_{\overset{arrow}{x}}^{q} = {\begin{bmatrix}0_{1{xq}} & 0_{1{xq}} & \ldots & 0_{1{xq}} \\{\beta_{1}( {x(1)} )} & {\beta_{2}( {x(1)} )} & \ldots & {\beta_{K}( {x(1)} )} \\\vdots & \vdots & \ddots & \vdots \\{\beta_{1}( {x( {N - q} )} )} & \ldots & \ldots & {\beta_{K}( {x( {N - q} )} )}\end{bmatrix}\mspace{14mu}{is}\mspace{14mu}{an}\mspace{14mu} N \times K\mspace{14mu}{matrix}}},{and}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$β_(k)(x(n)) is defined as:β_(k)(x(n))=|x(n)|^(k−1) x(n)and, {right arrow over (x)}==[x(1) x(2) . . . x(N)]^(T) an N×1 vectorrepresenting N samples of the input signal, and K and Q are the maximumpolynomial order and memory depth.

Referring to FIG. 8, there is shown an example of a multi-frequency MIMOsystem 800 in the form of a dual-band transmitter 820 having inputs 810and output 860. The dual-band transmitter 820 consists of low passfilters 830A and 830B, up-converters 835A and 835B, local oscillators(L.O.) 840A and 840B, and nonlinear transmitter 850. The input signalsare up-converted to carrier frequencies ω₁ and ω₂ from local oscillators840A and 840B using up-converters 835A and 835B. The up-convertedsignals from the up-converters 835A and 835B are combined by means of apower combiner 845 and are supplied, after combination, to the dual-bandtransmitter 850. The dual-band transmitter 850 exhibits nonlinear and/orlinear distortions (impairments) behaviors. The distortion behaviors mayinclude but not limited to nonlinear power response of the activedevices such as the power amplifier, frequency response and memoryeffect.

Referring to FIG. 9, there is shown the output signal of the dual-bandtransmitter 820 presented in FIG. 8. Due to nonlinear behavior of thedual-band transmitter 820, the output signal 900 of the transmitter 820consists of desired signals 910A and 910B at carrier frequencies ω₁ andω₂, intra-band distortions 920, and inter-band distortions 930A and930B.

Referring now to FIG. 10, there is shown a system 1000 comprising adigital multi-cell processing pre-compensator 1020 with dual inputs 1010and dual pre-distorted outputs 1015 cascaded in front of a dual-bandtransmitter 1030 similar to the one illustrated in FIG. 8 (withcomponents 1035A, 1035B, 1040A, 1040B, 1045A, 1045B, 1050 and 1060 ofFIG. 10 corresponding to components 830A, 830B, 840A, 840B, 835A, 835B,845 and 850 of FIG. 8). The digital multi-cell pre-compensator 1020 usesa matrix of two processing cells, 1025A and 1025B, in order tocompensate for the dual-band transmitter's nonlinearities and anyintra-band distortions (impairments) between the two RF signals. Thedigital multi-cell pre-compensator 1020 with dual inputs 1010 and dualoutputs 1015 comprises means, for example the processing cells 1025A and1025B, using the input signals 1010 and output signal 1070 of thedual-band transmitter 1030 to estimate any nonlinearities anddistortions (impairments) and identify a proper processing function foreach of the two processing cells PC1 1025A and PC2 1025B. Afteridentification, the input signals 1010 are supplied to the twoprocessing cells 1025A and 1025B to generate and adjust thepre-distorted signals 1015. Then the pre-distorted signals 1015 aresupplied to the dual-band transmitter 1030. The cascade of the digitalcompensator 1020 and the dual-band transmitter 1030 exhibits linearbehavior. The output signal 1070 is the linear amplified version of theinput signals 1010 without the effect of the transmitter'snonlinearities and intra-band distortions (impairments) on the qualityof the output signal. Therefore, the digital multi-cell processingpre-compensator block 1020 compensate for all the linear and nonlineardistortions (impairments) of the dual-band transmitter 1030.

Referring to FIG. 11, there is shown a system 1100 comprising a digitalmulti-cell processing pre-compensator 1120 with dual inputs 1110 andpre-distorted output 1150 cascaded in front of a multi-carriertransmitter 1160. The digital multi-cell pre-compensator 1120 uses amatrix of four processing cells, 1125A, 1125B, 1130A and 1130B, in orderto compensate for the multi-carrier transmitter's 1160 nonlinearitiesand any intra-band and inter-band distortions (impairments) between thetwo RF signals. The digital multi-cell pre-compensator 1120 comprisesmeans, for example the processing cells 1125A, 1125B, 1130A and 1130B,using the input signals 1110 and the output signal 1170 of themulti-carrier transmitter 1160 to estimate any nonlinearities anddistortions (impairments) and identify a proper processing functions foreach of the four processing cells PC1 1125A, PC2 11258, PC3 1130A, andPC4 11308. The processing cells PC1 1125A and PC2 1125B compensate forthe intra-band distortions and transmitter's nonlinearities aroundcarrier frequencies ω₁ and ω₂. The processing cells PC3 1130A and PC41130B compensate for the inter-band distortions at frequency bandscentered at 2ω₁-ω₂ and 2ω₂-ω₁. The pre-distorted output signals of theprocessing blocks are then up-converted to designated carrierfrequencies using the up-converters 1135A, 1135B, 1135C, and 1135D.Finally, the up-converted signals are combined in power combiner 1145and feed the input of the nonlinear multi-carrier transmitter 1160. Thecascade of the digital multi-cell pre-compensator 1120 and the dual-bandtransmitter 1160 exhibit linear behavior. The output signal 1170 is thelinear amplified version of the input signals 1110 without the effect ofthe transmitter's nonlinearities, inter-band, and intra-band distortions(impairments) on the quality of the output signal. Therefore, thedigital multi-cell pre-compensator 1120 compensates for all the linearand nonlinear distortions of the multi-carrier transmitter 1160.

Referring to FIG. 12, there is shown a system comprising a digitalpre-compensator 1220 with multiple inputs 1210 and multiple outputs 1250used for forward behavior modeling and simulation of thelinear/nonlinear behavior of multi-branches and multi-frequencies MIMOsystems. The digital pre-compensator 1200 is modeled as a N×N matrixwith N² cells 1230, with N inputs 1210 and N outputs 1250, where eachcell of the matrix represents a processing block. For example, D(i,j)represents the processing block where the input of the processing cellis the i.sup.th input 1210 of the MIMO system and the output of theprocessing cell is the input of the function ƒ_(j), which its output isthe j^(th) output 1240 of the digital pre-compensator 1220. Thefunctions ƒ_(i) 1240 can be modeled as linear or nonlinear functionswith/without considering the memory of the system.

Depending on the architecture of the MIMO system, the digitalcompensator with multiple inputs and multiple outputs 1220 can be addedbefore or after the MIMO system as pre-compensator or post-compensator.

Therefore, as taught by the above disclosure:

The pre-distorted multiple-input signal may be adjusted to introducelinear and nonlinear distortions on each signal path of themultiple-input signal to compensate for estimated impairments; and

The pre-distorted multiple-input signal may be adjusted to introduceinterference between each signal path of the multiple-input signal tocompensate for estimated impairments.

Each of the above described pre-processing cells may include nonlinearprocessing blocks compensating for multiple-input multiple-outputnonlinear distortions and an effect of interferences between signalpaths of the multiple-input signal and signal paths of themultiple-output signal. The nonlinear processing blocks process themultiple-input signal and the multiple-output signal to determine adesired multiple-output signal that pre-compensates for the nonlineardistortions; and estimating a nonlinear function for each nonlinearprocessing block.

Each of the above described pre-processing cells may include linearprocessing blocks compensating for multiple-input multiple-output lineardistortions and an effect of interferences between signal paths of themultiple-input signal and signal paths of the multiple-output signal.The linear processing blocks process the multiple-input signal and themultiple-output signal to determine a desired multiple-output signalthat pre-compensates for the linear distortions, and estimate a linearfunction for each linear processing block.

Each of the above described pre-processing cells of the matrix maycomprise nonlinear processing blocks compensating for multiple-inputmultiple-output nonlinear distortions and an effect of interferencesbetween signal paths of the multiple-input signal and signal paths ofthe multiple-output signal, and linear processing blocks compensatingfor the multiple-input multiple-output linear distortions and the effectof interferences between the signal paths of the multiple-input signaland the signal paths of the multiple-output signal. The non-linear andlinear processing blocks process the multiple-input signal and themultiple-output signal to determine a desired multiple-output signalthat pre-compensates for the non-linear and linear distortions, estimatea non-linear function for each non-linear processing block, and estimatea linear function for each linear processing block.

Each of the above described pre-processing cells of the matrix may modela behavior of multi-input multi-output system and may include anonlinear processing block to compensate for the multiple-inputmultiple-output system linear distortions and an effect of interferencesbetween signal paths of the multiple-input signal and signal paths ofthe multiple-output signal, and a linear processing block to compensatefor the multiple-input multiple-output system linear distortions and theeffect of interferences between the signal paths of the multiple-inputsignal and the signal paths of the multiple-output signal. Each of thenon-linear and linear processing blocks process the multiple-inputsignal and the multiple-output signal to determine a desiredmultiple-output signal that pre-compensates for the non-linear andlinear distortions, estimate a non-linear model for each non-linearprocessing block, and estimate a linear model for each linear processingblock.

Those of ordinary skill in the art will realize that the description ofthe system and methods for digital compensation are illustrative onlyand are not intended to be in any way limiting. Other embodiments willreadily suggest themselves to such skilled persons having the benefit ofthis disclosure. Furthermore, the disclosed systems can be customized tooffer valuable solutions to existing needs and problems of the powerefficiency versus linearity tradeoff encountered by designers ofwireless transmitters in different applications, such as satellitecommunication applications and base and mobile stations applications inwireless communication networks.

In the interest of clarity, not all of the routine features of theimplementations of signal pre-compensation processing mechanism areshown and described. It will, of course, be appreciated that in thedevelopment of any such actual implementation of the network accessmechanism, numerous implementation-specific decisions must be made inorder to achieve the developer's specific goals, such as compliance withapplication-, system-, network- and business-related constraints, andthat these specific goals will vary from one implementation to anotherand from one developer to another. Moreover, it will be appreciated thata development effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the field of telecommunication networks having thebenefit of this disclosure.

In accordance with this disclosure, the components, process steps,and/or data structures described herein may be implemented using varioustypes of operating systems, computing platforms, network devices,computer programs, and/or general purpose machines. In addition, thoseof ordinary skill in the art will recognize that devices of a lessgeneral purpose nature, such as hardwired devices, field programmablegate arrays (FPGAs), application specific integrated circuits (ASICs),or the like, may also be used. Where a method comprising a series ofprocess steps is implemented by a computer or a machine and thoseprocess steps can be stored as a series of instructions readable by themachine, they may be stored on a tangible medium.

Systems and modules described herein may comprise software, firmware,hardware, or any combination(s) of software, firmware, or hardwaresuitable for the purposes described herein. Software and other modulesmay reside on servers, workstations, personal computers, computerizedtablets, PDAs, and other devices suitable for the purposes describedherein. Software and other modules may be accessible via local memory,via a network, via a browser or other application in an ASP context, orvia other means suitable for the purposes described herein. Datastructures described herein may comprise computer files, variables,programming arrays, programming structures, or any electronicinformation storage schemes or methods, or any combinations thereof,suitable for the purposes described herein.

Although the present invention has been described hereinabove by way ofnon-restrictive illustrative embodiments thereof, these embodiments canbe modified at will within the scope of the appended claims withoutdeparting from the spirit and nature of the present invention.

The invention claimed is:
 1. A method for predistortion, the methodcomprising: receiving a plurality of input signals forming amultiple-input signal in a multiple-input multiple-output system;generating a pre-distorted multiple-input signal from the receivedmultiple-input signal; generating a multiple-output signal by feedingthe pre-distorted multiple-input signal into a multiple-input andmultiple-output transmitter; estimating impairments generated by themultiple-input and multiple-output transmitter, the impairmentscomprising nonlinear crosstalk between distinct ones of the plurality ofinput signals; and adjusting the pre-distorted multiple-input signal tocompensate for the estimated impairments, wherein generating thepre-distorted multiple-input signal comprises feeding the receivedmultiple-input signal to a matrix of pre-processing cells, comprising,in each of the pre-processing cells of the matrix: nonlinear processingblocks compensating for multiple-input multiple-output nonlineardistortions and an effect of interferences between signal paths of themultiple input signal and signal paths of the multiple-output signal;and linear processing blocks compensating for the multiple-inputmultiple-output linear distortions and the effect of interferencesbetween the signal paths of the multiple-input signal and the signalpaths of the multiple-output signal.
 2. The method of claim 1, whereinadjusting the pre-distorted multiple-input signal includes introducinglinear and nonlinear distortions on each signal path of themultiple-input signal.
 3. The method of claim 2, wherein adjusting thepre-distorted multiple-input signal further includes introducinginterference between each signal path of the multiple input signal. 4.The method of claim 1, comprising, in the nonlinear processing blocks:processing the multiple-input signal and the multiple-output signal todetermine a desired multiple-output signal that pre-compensates for thenonlinear distortions; and estimating a nonlinear function for eachnonlinear processing block.
 5. The method of claim 1, comprising, in thelinear processing blocks: processing the multiple-input signal and themultiple-output signal to determine a desired multiple-output signalthat pre-compensates for the linear distortions; and estimating a linearfunction for each linear processing block.
 6. The method of claim 1,comprising, in the non-linear and linear processing blocks: processingthe multiple-input signal and the multiple-output signal to determine adesired multiple-output signal that pre-compensates for the non-linearand linear distortions; estimating a non-linear function for eachnon-linear processing block; and estimating a linear function for eachlinear processing block.
 7. The method of claim 1, wherein each of thepre-processing cells of the matrix models a behavior of the multi-inputmulti-output system.
 8. A predistorter for a transmitter, comprising: amultiple-input for receiving a plurality of input signals forming amultiple-input signal; a matrix of pre-processing cells for generating apre-distorted multiple-input signal from the received multiple-inputsignal; and a multiple-output for feeding the pre-distortedmultiple-input signal to the multiple input and multiple-outputtransmitter; wherein the pre-processing cells are configured to estimateimpairments generated by the multiple-input and multiple-outputtransmitter and adjust the predistorted multiple-input signal tocompensate for the estimated impairments, the impairments comprisingnonlinear crosstalk between distinct ones of the plurality of inputsignals, wherein each of the pre-processing cells of the matrixincludes: nonlinear processing blocks compensating for multiple-inputmultiple-output nonlinear distortions and an effect of interferencesbetween signal paths of the multiple input signal and signal paths ofthe multiple-output signal; and linear processing blocks compensatingfor multiple-input multiple-output linear distortions and the effect ofinterferences between the signal paths of the multiple-input signal andthe signal paths of the multiple-output signal.
 9. The pre-compensatorof claim 8, wherein the adjustment of the predistorted multiple-inputsignal introduces linear and nonlinear distortions on each signal pathof the multiple-input signal.
 10. The pre-compensator of claim 9,wherein the adjustment of the predistorted multiple-input signal furtherintroduces interference between each signal path of the multiple-inputsignal.
 11. The pre-compensator of claim 8, wherein the nonlinearprocessing blocks are configured to: process the multiple-input signaland the multiple-output signal to determine a desired multiple-outputsignal that pre-compensates for the nonlinear distortions; and estimatea nonlinear function for each nonlinear processing block.
 12. Thepre-compensator of claim 8, wherein the linear processing blocks areconfigured to: process the multiple-input signal and the multiple-outputsignal to determine a desired multiple-output signal thatpre-compensates for the linear distortions; and estimate a linearfunction for each linear processing block.
 13. The pre-compensator ofclaim 8, wherein the non-linear and linear processing blocks areconfigured to: process the multiple-input signal and the multiple-outputsignal to determine a desired multiple-output signal thatpre-compensates for the non-linear and linear distortions, respectively;for the non-linear processing blocks, estimate a non-linear function foreach nonlinear processing block; and for the linear processing blocks,estimate a linear function for each linear processing block.
 14. Thepre-compensator of claim 8, wherein each of the preprocessing cells ofthe matrix models a behavior of a multi-input multi-output system.